1. Field of the Invention
The present invention relates to an apparatus for applying a liquid or viscid coating medium onto a moving material web, especially one made of paper or cardboard.
2. Description of the Related Art
An exemplification of a known apparatus for applying a liquid or viscid coating medium onto a moving material web is illustrated in FIG. 2a. The coating apparatus includes an applicator roll 12 and a counter roll 14. The two rolls, 12 and 14, form a nip through which material web 16 runs in the direction of feed. Applicator roll 12 is coated with a liquid or viscid coating medium by use of a coating mechanism 18. This medium is subsequently transferred onto the material web 16 in the area of the coating location S. Applicator roll 12 and counter roll 14 are driven in opposite directions around their respective axes A and B as indicated by arrows P and P'. Such coating apparatuses are applied, for example, in film coating processes, roll coating processes and size presses.
Coating apparatus 10 usually experiences problems in the area Sp--located adjacent to the coating location S, on the downstream side relative to the movement of the material web 16--as the material web 16 separates from the outer surface 12a of roll 12. The pre-metered coating of coating medium 20 being applied onto the outer surface 12a is only partially transferred in the nip onto the material web 16. A part 20b of the coating medium 20 adheres to the outer surface of roll 12, and therefore, returns back to the coating mechanism 18. Those experienced in the art, refer to this effect as "film splitting".
In the process of film splitting, "coating medium threads" F (FIG. 2b) develop between the coating medium 20 being applied onto the material web 16 and the remaining layer 20b of coating medium on roll 12--similar to the honey threads that are formed between a spoon that is pulled from a honey jar. When these threads F are torn, it is possible to develop droplets or particles 20c of the coating medium 20, which is reflected in the formation of spray mist N. Accordingly, it can be said that the spray mist is formed by the particles and droplets 20c of the coating medium 20, which, for example, are torn from the previously applied coating 20a by the adhesive force of applicator roll 12.
Due to the aforementioned formation of the spray mist, the quality of the coated material web can be compromised in several ways. For example, small craters can form on the outer surface of the coated material web. Furthermore, the remaining parts of the torn-off coating medium threads F can cause a so-called "orange skin effect". Additionally, the re-deposit of droplets or particles 20c in an area removed from the spray area Sp can lead to an irregular structure of the material web's coating surface. The spray mist also causes other drawbacks such as a contamination of the coating apparatus.
The risk of encountering the above-stated disadvantages increases as the feed speed of the material web 16 increases, as the area specific coating weight on the material web 16 increases, and as the absorbency capability and/or the bibulousness of the material web 16 decreases. To avoid the aforementioned disadvantages of the state of the art, it was suggested in a summary report issued by the "Tappi Metered Size Press Forum" by Dr. Hans W. Maurer (weekly publication for paper manufacturing 17, 1997, page 827) to vary the composition of the coating medium. This approach, however, has proven to be impractical.